One of the problems faced by the perfumery industry lies in the relatively rapid loss of the olfactive benefit provided by odoriferous compounds due to their volatility, particularly that of “top-notes”. This problem is generally tackled using a delivery system, e.g. capsules containing a perfume, to release the fragrance in a controlled manner.
A key requirement from the industry regarding these systems is to provide a good olfactive experience in a challenging medium such as a perfumed consumer product. This means, on one hand, that efficient delivery systems should survive in challenging consumer product bases containing high level of surfactant without physically dissociating or degrading. This performance corresponds to the stability of the delivery systems. On the other hand, another success criteria for delivery systems encapsulating actives such as a perfume consists of the efficiency in releasing the perfume referred to as olfactive performance and related to the way capsules are breaking during use of the consumer product.
Aminoplast microcapsules formed of a melamine-formaldehyde resin have been largely used to encapsulate hydrophobic actives, thus protecting said actives and providing their controlled release. Although capsules such as aminoplast ones afford good olfactive performance, they suffer from stability problems when used in consumer products comprising surfactants, such as perfumery consumer products, especially after prolonged storage at elevated temperatures. In such products, even though the capsule wall remains intact, the encapsulated active tends to leak out of the capsule by diffusion through the wall due to the presence of surfactants that are able to solubilise the encapsulated active in the product base. The leakage phenomenon reduces the efficiency of the capsules to protect the active and provide its controlled release.
A variety of strategies have been described to improve the stability of oil core-based microcapsules. Cross-linking of capsule walls, with chemical groups such as polyamines and polyisocyanates, has been described as a way to improve stability of microcapsules. WO 2011/154893 discloses for instance a process for the preparation of polyurea microcapsules using a combination of aromatic and aliphatic polyisocyanates in specific relative concentrations and wherein the most preferred polyamine is selected from the group consisting of guanidine, water-soluble guanidine salts and N,N′-bis-(3-aminopropyl)-ethylenediamine. Compared to aminoplast, polyurea-based microcapsules present the additional advantage of being free from melamine-formaldehyde. However, these capsules are not always satisfactory in terms of mechanical properties as that are less friable, which can negatively impact their olfactive performance represented by the odor intensity perceived during handling and after intentional breakage e.g. by rubbing.
A solution to the above-mentioned problem has been proposed in WO 2013/000587. In particular, an improvement of the breakability of the polyurea microcapsules and so of the olfactive performance of the polyurea microcapsules while maintaining a satisfying stability in consumer product has been described by replacing guanidine and guanidine salts, commonly used as polyamines in such capsules, by 3,5-diamino-1,2,4-triazole.
However, there is still a need to provide improved capsules to those known from the prior art, that would be free of melamine-formaldehyde and at the same time olfactively more performing than existing polyurea-based capsules but also as stable as existing melamine-formaldehyde and/or polyurea-based capsules in challenging media such as surfactant-based consumer products.
The present invention provides a solution to the above mentioned problems through a process wherein the shell of microcapsules is formed by the reaction between at least one aromatic polyisocyanate and a mixture of two polyamines. The use of two polyamines to form microcapsules has been reported in US2013/313734. Said document points out the importance to add first a guanidine aqueous solution to form a pre-microcapsules and then add a second polyamine.